As electronic devices increasingly become portable, advances must be made in energy storage systems to enable such portability. Indeed, it is often the case with current electronic technology that the limiting factor to portability of a given device is the size and the weight of the associated energy storage device. A small energy storage device, such as a battery, may be fabricated for a given electrical device, but size decreases come at the cost of energy capacity. Conversely, a long lasting energy source can be built but it is often too large or too bulky to be comfortably portable. The result is that the energy source is either too heavy or does not last long enough for a particular user's application.
Numerous different battery systems have been proposed for use over the years. Early rechargeable battery systems included lead acid, and nickel cadmium (NiCad), each of which has enjoyed considerable success in the market place. Lead acid batteries are preferred for applications in which ruggedness and durability are required and hence have been the choice of automotive and heavy industrial settings. Conversely, NiCad batteries have been preferred for smaller portable applications. More recently, nickel metal hydride systems (NiMH) have found increasing acceptance for both large and small applications.
Notwithstanding the success of the foregoing battery systems, other new batteries are establishing a reputation for better capacity, better power density, longer cycle life, and lower weight, as compared with the current state of the art. The first such system to reach the market is the lithium ion battery, which has found its way into numerous consumer products. Lithium polymer batteries have also received considerable attention, although they are only now beginning to reach the market.
Lithium ion batteries in general include a positive electrode fabricated from, for example, a transition metal oxide material, and a negative electrode fabricated from an activated carbon material such as graphite or petroleum coke. New materials for both electrodes have been investigated intensely because of the high potential for improved energy density. Typically the positive and negative electrodes are permeated by a shared electrolyte medium, and are held in close proximity at a uniform distance from each other so as to minimize cell polarization while maximizing the uniformity and efficiency of capacity utilization across the cell. To prevent short circuits and yet allow ion migration across the cell, a thin plastic microporous membrane is commonly placed between the negative and positive electrodes of lithium ion cells.
Typically the electrode active materials are deposited on electrically conducting substrates, also called current collectors, and for reasons of electrochemical stability, the cathode substrate is typically aluminum foil and the anode foil is typically copper foil, though other materials such as carbon substrates could be selected if desired. However, the choice of support media for cell electrodes does not mean that ideal combination of characteristics for service in that capacity have been obtained from aluminum, copper or carbon. Adhesion of electrode coatings to substrates is sometimes quite weak, and copper in particular presents difficulties for workers attempting to bond coatings to it. Adhesion is, of course, dependent upon the degree of oxidation of the surface of the substrate, the cleanliness of the supplied substrate, the choice of the binder used for the active material, as well as roughness and other surface characteristics of active material particles. A poorly adhering coating can lift off of a substrate. This lift-off disrupts the homogeneity of electrical resistance (and thereby current flow) across the surface of such an electrode. Furthermore, if lift-off occurs at the anode of a cell based on lithium ion chemistries, it can lead to lithium plating, dendrite formation and possible hazardous internal short-circuits.
Such lift-off is unusual in conventional lithium ion cells. For those cells, the pressure exerted to maintain the cell in the preferred dimensions is referred to as "stack pressure" due to the serial arrangement of cells either as flat stacks or as a single cell wound as an evenly coiled spool in a circular "stack". In commercial lithium ion cells, stack pressure is typically enforced by placing a tightly rolled ("jelly roll") cell into a rigid metal can (often cylindrical in shape) with internal dimensions that are only slightly larger than the full size of the "jelly roll". Recently, however, the industry has been developing cells housed in lighter and more flexible packaging materials, for instance employing a heat-sealable plastic material such as a thin foil bonded in a sandwich to layers of plastic sheeting. Unlike the rigid cans, this new packaging format cannot be expected to provide sufficient rigidity and strength to maintain the stack pressure of the cell. Thus lift-off of an electrode coating and consequent deterioration of performance can occur in cells with flexible packaging.
Accordingly, there exists a need for electrode compositions with improved adhesion to metal surfaces.